The geographic distribution of organisms on Earth follows patterns that are best explained by evolution, in combination with the movement of tectonic plates over geological time. For example, broad groupings of organisms that had already evolved before the breakup of the supercontinent Pangaea (about 200200200 million years ago) tend to be distributed worldwide. In contrast, broad groupings that evolved after the breakup tend to appear uniquely in smaller regions of Earth. For instance, there are unique groups of plants and animals on northern and southern continents that can be traced to the split of Pangaea into two supercontinents (Laurasia in the north, Gondwana in the south).
Marsupial mammals on Australia likely evolved from a common ancestor. Because Australia's has remained isolated for an extended period time, these mammals have diversified into a variety of niches (without being outcompeted by placental mammals).
The evolution of unique species on islands is another example of how evolution and geography intersect. For instance, most of the mammal species in Australia are marsupials (carry young in a pouch), while most mammal species elsewhere in the world are placental (nourish young through a placenta). Australia’s marsupial species are very diverse and fill a wide range of ecological roles. Because Australia was isolated by water for millions of years, these species were able to evolve without competition from (or exchange with) mammal species elsewhere in the world.
The marsupials of Australia, Darwin's finches in the Galápagos, and many species on the Hawaiian Islands are unique to their island settings, but have distant relationships to ancestral species on mainland. This combination of features reflects the processes by which island species evolve. They often arise from mainland ancestors – for example, when a landmass breaks off or a few individuals are blown off course during a storm – and diverge (become increasingly different) as they adapt in isolation to the island environment. What is geography?
